The present invention relates to a metallic mold for tire curing, which includes a plurality of pieces used to form a tire tread pattern by sections, and a process for producing the same. More specifically, the invention relates to a metallic mold for tire curing, which is capable of improving an appearance of a produced tire by preventing spew formation, and also capable of reducing the production of defectives during curing and molding, and a process for producing the same.
Conventionally, regarding a sector mold used as one of metallic molds for tire curing, a sector is formed by arranging a plurality of pieces in an adjacent relation to each other, the pieces having concave parts and convex parts corresponding to a profile of a tread section of a produced tire, and then fixing these pieces to a back block. Then, an annular sector mold is made by interconnecting a plurality of such sectors.
It has been known that during tire curing and molding, air is left or gas is generated between an outer peripheral surface of the tire and a molding surface of the metallic mold to form very small concaves in the surface of the produced tire and, consequently, a defective product is produced. To deal with this problem, an air vent through-hole called a vent hole or an exhaust groove called a vent groove has conventionally been formed in the molding surface of the metallic mold.
However, a rubber material flows into the vent hole or the vent groove during tire curing and molding to form spews (or flashes: whisker-like protruding portions) in the surface of the produced tire. This necessitated finishing work to be carried out for removing the spews after molding. But even the removal of the spews didn""t erase traces thereof. Thus, an appearance of the produced tire was damaged, and tire noises were increased.
Under these circumstances, in recent years, in order to improve an appearance of the tire and reduce the production of defectives during tire curing and molding, a spewless mold including a slit vent composed of a very small gap has been made available without forming any vent holes or vent grooves in the molding surface.
Regarding such a spewless mold, for example, the followings have been presented: {circle around (1)} Japanese Patent Laid-Open Hei 4 (1992)-223108 (Title of the Invention: METALLIC MOLD FOR CURING UNCURED TIRE CARCASS, PROCESS FOR PRODUCING THE SAME, AND TIRE CURING PROCESS USING THE SAME); and {circle around (2)} Japanese Patent Laid-Open Hei 10 (1998)-24423 (Title of the Invention: METALLIC MOLD FOR TIRE CURING, AND PROCESS FOR PRODUCING THE SAME).
The process disclosed in the former invention {circle around (1)} is for assembling back blocks for forming a tire tread pattern by sections divided in a pitch, and then sucking air existing between the tire and a metallic mold through a gap provided between back blocks. The process disclosed in the latter invention {circle around (2)} is for casting a matrix metal (aluminum) to form a pattern piece around a core piece of a different kind of metal (SS400), and then discharging air through a gap formed at a tire curing temperature (160xc2x0) based on a difference in a linear thermal expansion coefficient between the metals. In addition, for a slit vent mold, there is available a method of venting air by providing a level difference between pieces by means of machine work or inserting a spacer between the pieces.
However, in the case of the invention {circle around (1)}, it is difficult to secure a proper air vent in an optional position, and it is always necessary to install a vacuum device or the like as exhaust means. In the case of the invention {circle around (2)}, a sufficient volume must be provided for forming a gap based on a difference in a linear thermal expansion coefficient between the metals, and it is difficult to secure a necessary gap quantity in an optional position. Thus, it is difficult to substantially vent air from the metallic mold.
It is an object of the present invention to provide a metallic mold for tire curing, which is capable of facilitating formation of a gap having necessary exhaust performance in an optional position in a molding surface, thus preventing spew formation to improve an appearance of a produced tire, and also capable of reducing production of defectives during curing and molding. It is another object of the invention to provide a process for producing the metallic mold.
In order to achieve the above object, in accordance with the present invention, a metallic mold for tire curing comprises a plurality of pieces for forming a tread pattern of a tire by sections, the pieces being fixed to a back block and arranged in an annular form. In this case, each of the pieces is composed of a plurality of piece block layers laminated by performing a plurality of shots of die casting for metallic materials, and a very small gap is formed in a joined portion between the piece blocks by means of coagulation and contraction of the metallic materials.
In accordance with the present invention, a process for producing the tire curing metallic mold comprises the steps of:
performing a first shot of die casting to cast a molten metallic material in a first metallic mold, and forming a first piece block having a partial molding surface corresponding to a tread pattern of a tire; performing a second shot of die casting to cast the same kind of a molten metallic material in a second metallic mold while the first piece block is arranged in the second metallic mold, and forming a second piece block on the first piece block so as to complement the molding surface; and forming a very small gap in a joined portion between the first and second piece blocks by means of coagulation and contraction of the metallic materials.
With the invention, during casting and laminating the plurality of piece block layers constituting each piece by using the same kind of metallic materials, a very small gap is formed in a joined portion between the piece blocks by means of coagulation and contraction of the metallic materials. This very small gap formed by coagulation and contraction of the metallic materials exhibits exhaust performance for discharging only air or gas remaining in the metallic mold well without allowing a rubber material to flow out. Accordingly, a gap having necessary exhaust performance for a spewless mold can be easily formed in an optional position of the molding surface based on the arrangement of the joined portion, and the production of defectives can be reduced during curing and molding while preventing spew formation.
Moreover, a tire appearance can be improved without performing spew removal after tire curing or without leaving any spew traces on a tire surface, the spew traces causing deteriorated tire noises.
In accordance with the present invention, a pneumatic tire comprises a tread pattern having no spew traces, which is formed on a tire surface by using the foregoing metallic mold for tire curing.
Furthermore, in accordance with the present invention, a process for producing a pneumatic tire comprises the step of forming a tread pattern having no spew traces on a tire surface by using the foregoing metallic mold for tire curing.